1. Field of the Invention
This invention relates to novel liquid crystal compounds and a liquid crystal composition containing the same. More particularly it relates to chiral liquid crystal compounds having an optically active group and a chiral liquid crystal composition containing the same.
2. Description of the Related Art
As liquid crystal display elements, those of TN (Twisted Nematic) type display mode have currently been most broadly used, but they have been inferior to emissive type display elements (such as those of electroluminescence, plasma display, etc.) in the aspect of response speed. Thus, various improvements in this respect have been attempted, but it does not appear that the improvement to a large extent has been achieved. Accordingly, various liquid crystal display devices based on other principles in place of TN type display elements have been attempted, and as one of such devices, there is a display mode utilizing ferroelectric liquid crystals (N.A. Clark et al, Applied Phys. Letter, 36, 899 (1980)). This mode utilizes the chiral smectic C phase (hereinafter abbreviated to SC* phase) or chiral smectic H phase (hereinafter abbreviated to SH* phase) of ferroelectric liquid crystals, and those having these phases in the vicinity of room temperature are preferred.
Some liquid crystal compounds exhibiting chiral smectic phases have already been known for example, those of carbonic acid esters disclosed in Japanese patent application laid-open No. Sho 60-54341/1985, that is, those expressed by the formula ##STR2## wherein R represents a linear chain or branched alkyl group of 1 to 18 carbon atoms; l represents 0 or 1; m represents 1 when l=0, and represents 1 or 2 when l=1; X represents COO or --O-- when m=1 and represents a single bond when m=2; and * represents an optically active carbon, but there has still been no compound exhibiting satisfactory properties.